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Abstract

Compact filters and demultiplexers based on long-range air-hole assisted subwavelength (LR-AHAS) waveguides have been proposed and numerically demonstrated. The tunable reflective filters possess the characters of high extinction ratio (17.5dB) and narrow bandwidth (10.1nm). The average demultiplexing bandwidth of a 1 × 3 wavelength demultiplexer based on LR-AHAS waveguide is 17.3 nm. The drop efficiencies can be significantly enhanced up to 60% by employing proposed filters in the structure. With distinguished wavelength-filtering/dropping characters and compact footprints, the proposed filters and demultiplexers could become the fundamental signal processing components in the LR-AHAS waveguides for large-scale photonic integrations.

Figures (8)

(a) A drawing of the LR-AHAS waveguide array with five channels. Inset at the left top is
the contour profile of the field Ey (TE-mode) transmitted in the LR-AHAS waveguide at
λ = 1.55 μm. The first and fourth channels counted from the left are wave
guiding ones. The second, third, and fifth channels are embedded with proposed reflective
filters (indicated by dashed boxes), in which the corresponding resonant light with tunable
wavelength can be highly reflected back. (b) Schematic of the in-line stub-like (ILSL) filter
(region) corresponding to dashed part in the fifth channel. (c) Transmission spectra of the
ILSL filters with silver sidewalls (solid line) and without them (dashed line). Inset is the
structure of the ILSL filter without silver sidewalls, where re =
0.38•Period, and rs = 0. (d) The contour
profiles of field Ey of the ILSL filter at the wavelengths of 1.624 μm and 1.550
μm corresponding to the solid line in the spectra.

(a) The resonant wavelength versus the variation of radius of one air-hole in the cavity
(ΔL) and versus the width of the stub (ΔW).
(b) The evolution of the standing wave pattern in the ILSL filter, which is the direct proof
that two silver-dielectric interfaces act as two reflection walls of the cavity and the
directions of k-vectors of resonant light and incident light are almost orthogonal. (c)
Transmission spectra of the ILSL filters with three different rs
(0, 0.1•Period and 0.15•Period) while
re is fixed at 0.38•Period and
0.41•Period for the six curves, respectively. (d) Transmission
spectra of the ILSL filters with five different re while removing
the defective air-hole from the ILSL region (rs = 0).

(a) Schematics of a single in-line stub-like channel drop filter (ILSL CDF) and a combined
unit consisted of an ILSL CDF and an ILSL filter, which are indicated by the black and blue
dashed boxes in the lower structure. The directions of drop and incident light are
perpendicular. Based on the resonant tunneling effect, the combined unit can enhance the drop
efficiency compared to the single ILSL CDF. (b) The dropping wavelength (black curve with
circles) and the corresponding value of FWHM (blue circles) versus the radius of nine
enlarged air-holes in the black dashed box of a single ILSL CDF. (c) The transmittance of a
single ILSL CDF and a combined unit with re =
0.38•Period and rs = 0 versus the
thickness of the gap and versus the center-to-center separation S,
respectively.

(a) Schematic of a 1 × 3 wavelength demultiplexer consisted of three different ILSL
CDFs acting as demultiplexing units indicated by the black dashed boxes in the LR-AHAS
waveguide. Each demultiplexing unit can be replaced with the corresponding combined unit to
enhance the drop efficiency based on the resonant tunneling effect. (b) Transmission spectra
of three dropping channels with and without the ILSL filters (or enhancement). (c) The
contour profiles of field Ey of the 1 × 3 wavelength demultiplexer at the
corresponding dropping wavelengths from the first channel to the third one: 1.675 μm,
1.614 μm, and 1.554 μm.

(a) The equivalent waveguide (right one), in which all air-holes are replaced with the
square-shape material with gradual index change from 1.3 to 3.5, considering the
Bragg-effect along the propagation direction. (b) A further approximation that defines
homogeneous and effective relative permittivity εeff of
regions I and II to the left equivalent waveguide, for operational wavelengths far away from
the resonant wavelengths of Bragg-effect.

(a) Real and (b) imaginary parts of the effective index for the corresponding MDM
waveguide (blue line with triangular dots) and the equivalent waveguide corresponding to the
right one in Fig. 6(b) (red line with round
dots).

Silicon waveguide isolation versus the separation. Inset is the comparison of integration
density between silicon and LR-AHAS waveguide arrays with the same waveguide isolation of
50.3 dB under the same plotting scale.